I. Field of the Invention
The present invention relates generally to hemming methods and hemming machines of the type used in the automotive industry to hem a sheet metal panel over an inner panel to form a rigid assembly.
II. Description of Related Art
In the automotive industry, hemming machines are conventionally used to attach two metal panels together. These metal panels include, for example, the metal panels to form the automotive vehicle hood, door panels, and the like.
In the previously known hemming methods, a substantially 90 degree flange is first formed on an outer panel. Thereafter, an inner panel is positioned upon the outer panel so that an outer edge of the inner panel lies adjacent the bend line for the flange.
A prehemming tool then contacts and compresses the flange such that the flange overlies the outer edge of the inner panel. Typically, the prehemming tool bends the flange at a 45 degree angle relative to the plane of the outer edge portion of the inner panel.
Thereafter, a generally planar final hemming tool compresses the prehemmed flange against the inner panel so that the outer edge portion of the inner panel is sandwiched in between the flange and the outer panel thus securing the inner and outer panels together.
The previously known hemming methods and the machines for performing those methods, however, suffer from a number of disadvantages. One disadvantage of these previously known hemming methods and the machines for performing those methods is that three distinct machining operations are necessary to complete the hemming operation. These three machining operations include a flanging operation for initially forming the flange on the outer panel, a prehemming operation to bend the outer panel flange so that it overlies the outer edge portion of the inner panel and, finally, the final hemming operation to compress the flange against the outer edge portion of the inner panel. The necessity of three distinguishing operations inherently increases the machining cost for the final body panel. Furthermore, in many cases a separate flanging machine is used to form the flange on the outer body panel while a different machine performs both the prehem and final hem operations. The requirement to have two distinct machines, i.e. a flanging machine and a hemming machine, further increases the overall manufacturing cost of the body panel.
A still further disadvantage of these previously known hemming methods and the machines for performing these methods is that a relatively large amount of power is required during the final hemming operation to adequately compress the flange against the inner panel. The actuators as well as the components associated with the actuators to achieve this high power during the final hemming operation also increase the overall cost of the hemming machines, their installation cost as well as their energy consumption.
A still further disadvantage of these previously known hemming methods and the machines for performing those methods is that the relative movement between the prehemming and final hemming tools and the body panel assembly is in a direction generally perpendicular to the plane of the body panels. In some applications, however, there is simply insufficient room in the direction perpendicular to the plane of the inner panel to accommodate such movement of the prehemming and final hemming tools. For example, in an automotive roof opening, such as a moon roof or sun roof, a portion of the inner body panel is typically positioned close to and immediately beside the outer edge portion of the inner body panel. In this situation, the conventional prehemming and final hemming methods for forming the hem cannot be used.
In order to overcome this limitation of the conventional hemming methods, one specialized hemming method and apparatus for performing the hem particularly suited for roof openings in automotive vehicles is disclosed in U.S. Pat. No. 6,035,504. In the '504 patent, the inner and outer panels are arranged so that the coupon on the outer panel protrudes outwardly from the outer edge of the inner panel. The flanging side of the tool then first forms a substantially 90 degree flange on the coupon by deflecting the coupon laterally with respect to the inner body panel which is maintained in position by a back-up steel, and so that the outer body panel forms a bend line at its contact point with the inner body panel. Following the flanging operation, the prehemming side of the tool contacts the flange and bends the flange such that the flange overlies and is in close proximity to the outer body portion on the inner body panel. Finally, following the prehemming operation, the final hemming side of the tool compresses the flange against the inner body panel thus completing the hemming operation.
The main disadvantage of this tool is that a large gap is required to move the back-up steel into position to firmly maintain the upstanding inner flange when forming the initial flange on the outer coupon.
One disadvantage of this previously known hemming method, however, is that the flange on the outer body panel is in contact with the outer edge of the inner body panel following the flanging operation. Consequently, during the subsequent prehemming and hemming operations, a compression load is imposed on the outer edge of the inner body panel during the prehemming operation. When this occurs, distortion of the inner body panel and/or distortion of the outer body panel can result.
A still further disadvantage of this previously known hemming method is that the final hemming tool, during the final hemming operation, compresses the flange against the inner body panel by movement of the final hemming tool in a direction generally perpendicular to the inner body panel thus compressing the flange against the inner body panel. Performing a satisfactory hem using perpendicular compression during the final hemming operation, however, requires a relatively large amount of power for the hemming machine.